1. Technical Field
This invention relates generally to powder metallurgy. More particularly, the invention relates to sintered iron-based powder metal alloy articles that are suitable for use in high-wear applications. Most particularly, the invention relates to sintered iron-based powder metal articles, such as valve train components, including cam lobes and other valve components.
2. Related Art
The valve train of an internal combustion engine typically includes one or more camshafts. Camshafts for piston-driven internal combustion engines typically include several cam lobes with lobe-shaped outer surfaces that operate to move push rods, lifters or other movable members in a precise pattern. As the camshaft rotates, the cam lobes must engage the movable members at proper positions and with proper timing. Therefore, the cam lobes must be positioned on the camshaft at precise relative axial positions and angular orientations. Camshafts and their associated cam lobes are examples of components that are subject to sliding wear processes. These components have been produced by machining from unitary cast, forged or bar stock material. Recently, there has been a trend towards the use of assembled camshafts in order to reduce weight and offer design flexibility with respect to material selection for the high-wear surfaces and components, such as cam lobes, bearings, and other components. Assembled camshafts have been recognized as offering a cost-effective alternative as compared to traditional machined camshafts, as well as offering improved product quality and performance characteristics. Currently, the main application of assembled camshafts is in valve trains with roller followers, which require high fatigue strength in rolling contact. Cam lobe materials used in such applications are produced by forging of various types of cast or bar stock blanks, as well as powder forging and sintering. Assembled camshafts are not typically used for application in valve trains with sliding followers. Assembled camshafts are not used for sliding applications due to the tribologicial incompatibility between current cam lobe materials and the follower (tappet shim) material. This incompatibility results in the scuffing/pitting of the cam lobe and the follower.
In valve trains with sliding followers, cast camshafts are used, and in particular cast camshafts made using chilled cast iron. The superiority of chilled cast iron (CCI) over alternative materials such as hardenable steel when used under sliding contact conditions in traditional valve train designs has been proved. The use of chilled cast iron cam lobes for assembled camshaft applications has been considered, but generally has not been utilized because of limitations associated with the accuracy of the cast cam lobe components and the necessity of utilizing relatively expensive secondary machining operations to obtain the necessary dimensional accuracy of the finished cam lobes. However, the expanded use and development of multi-valve engines necessitates the use of camshafts with more design flexibility, including high wear resistance, assembled camshaft as opposed to unitary cast camshaft construction and near net shape forming of precision elements, such as cam lobes.
In order to fulfill these requirements, the use of powder metal technology has been considered for manufacturing portions of the camshaft from subassembly. However, less than fully dense powder metal components (i.e., those which are not sintered together with the application of pressure or the use of specialized sintering techniques to obtain full density, such as liquid phase sintering) have been unable to achieve the wear performance of chilled cast irons. Successful applications of powder metal alloys in sliding applications have been reported in U.S. Pat. No. 4,243,414 and UK Patent 2,187,757. These patents teach the use of highly alloyed powder metal compositions that are sintered to almost full density via liquid phase sintering. Another example of the reported successful use of powder metal technology in manufacturing cam lobes is disclosed in SAE Publication No. 960302 by Yoshikatsu Nakamura et al. which teaches the use of an Fe—C—P—Ni—Cr—Mo liquid phase sintered alloy to obtain higher pitting and scuffing resistance. As seen from the above examples, the related art teaches the use of highly alloyed materials and specialized sintering techniques, such as liquid phase sintering, in order to achieve high wear resistance.
Alloying with chromium from a mixture of iron, an iron chromium intermetallic compound and carbon is disclosed in U.S. Pat. Nos. 3,698,877, 5,476,632 and 5,540,883. U.S. Pat. No. 3,698,877 teaches a process of making high density parts by mixing iron with carbon and a brittle FeCr in a so-called sigma phase. U.S. Pat. Nos. 5,476,632 and 5,540,883 teach a process of forming a sintered component by blending carbon, a ferro chromium alloy powder and lubricant with compressible elemental powder, pressing the blended mixture to form the article, and then high temperature sintering of the article in a reducing atmosphere or under a vacuum. In these patents, emphasis is placed on alloying with Cr, Mo and Mn through addition of elemental ferro alloys or master alloys in order to achieve high strength without loss of compressibility for the powder mixture, or loss of formability for the as-sintered component. The process described in these patents is designed to produce a homogeneous Cr—Mn—Mo steel through high temperature solid diffusion in a vacuum furnace of elemental alloying elements. Two main groups of alloys are described in the above patents: 1) a group of Mn containing alloys for high strength applications (i.e., Fe—Mn—Mo—Cr—C), and 2) a group of Mn-free alloys for high ductility and post sintering forming operations (i.e., Fe—Mo—Cr—C). In both cases, carbon is added in powder form before compaction. The carbon ranges between 0.1 to 0.6% by weight, and is not sufficient for forming carbides with the alloying elements.
Therefore, it is desirable to develop sintered powder metal alloy materials which may be utilized to make cam lobes for assembled camshaft applications, as well as other high-wear applications, which may be formed to a near-net shape, and which do not require significant secondary machining or other finishing operations, and which do not have the disadvantages of related art sintered powder alloy materials.